M. Heydari-Malayeri - Paris Observatory

A class of → O-type stars whose spectra show
He II 4686 absorption line much stronger than any other He line especially
He II 4541 or He I 4471. Vz stars are thought to be
young stars lying close to the → ZAMS
(Walborn & Parker 1992, ApJ 399, L87). Alternatively, the Vz characteristics may be
related to the wind properties rather than to the
youth of the star (Martins et al., 2005, A&A 441, 735).

A member of a class of → pulsating stars with a period of 1 to 35 days
located in the → instability strip of the
→ H-R diagram. Also known as type II Cepheid variables,
W Virginis stars are typically 1.5 mag fainter than classical Type I Cepheids and have
a mass less than that of the Sun.
They also exhibit a period-luminosity relation which is distinct, but works in
a similar way to the relation for Type I Cepheids. Hence W Virginis stars can also
be used to measure Galactic and extragalactic distances.

An object showing signatures of both → WN Wolf-Rayet stars
and → WC Wolf-Rayet stars
in the same spectrum, originating from individual stars rather than from WN + WC
→ binary systems. Such a WN/WC signature implies
that the surface composition of the star is → nitrogen
and → carbon enriched. This hybrid state
results from a mixing process between He-burning
→ convective core and the
overlying nitrogen enriched layers. According to model predictions, this situation corresponds to a
short transition phase, lasting 103 to 104 years, during which
a WN star evolves into a WC star.
The WN/WC stars
so far detected are all of early types.

A new type of → Wolf-Rayet stars found in the
→ Large Magellanic Cloud (LMC).
These stars have both strong → emission lines,
as well as → He II lines
and → Balmer lines in absorption
and spectroscopically resemble a → WN3
and → O3V binary pair. However, they are visually too
faint to be WN3+O3 V → binary systems.
So far nine WN3/O3 types have been
detected, making up ~ 6% of the population of LMC WRs. Their
temperatures are estimated to be around 100,000 K, a bit hotter than the
majority of → WN Wolf-Rayet
stars (by around 10,000 K) although a few hotter WNs
are known. The abundances are what you would expect for
→ CNO equilibrium. However, most anomalous are their
→ mass-loss rates which are more like that of an
→ O star than a WN star. While their
evolutionary status is uncertain, their low mass-loss rates and
→ wind velocities
suggest that they are not products of homogeneous
evolution. It is possible instead that these stars represent an
intermediate stage between O stars and WNs. Since WN3/O3 stars are
unknown in the Milky Way, their formation would depend upon
→ metallicity
(Neugent et al., 2017, arxiv:1704.05497).